OK, as a recovering sailor...... a head sail is forward of the mast. Usually a jib. Be it a staysail, 100 (working jib)/110/135/150 (genoa)and so on. (100 percent is the distance from the tack, that's where the bottom forward corner attaches to the boat.... anything over 100% is the percentage of the foot, that's the bottom side of the triangle, extends past the mast) Then there's bloopers, spinnakers and other variations of downwind sails. (As in sailing on a broad reach or running, which is....but I digress.....)
Why yes I do have some experience, with a slew of sailboats over the last 50 years, but more to the point, recently a 37 foot motor sailor where we collected just these kind of data.
So here's the answer to your question. If you get a LARGE jib up there..... say 300+ square feet and it's blowing on the beam or abaft.... at, oh, 15 or so knots..... you will add 1-2 knots. In reality what you will do is pull back the throttle, because you don't want to waste any added energy pushing hull speed. So.... WAG 7 knots at 1450 rpm would become 7 knots at 1050.......Burn would be less. If your a twin, you may want to consider shutting one down. (Caution, depending on your tranny this may require a shaft brake)
So a sail may help for economy..... it's not going to turn it into a Cigarette.
There are plenty of used sail lofts. Even if you can't find the exact size, get something bigger for not too much, and have them cut it down.
Sailors change sails like power boaters change props..(chasing the same dream.... more for less)... and the cost is about the same. Always used sails around.
I hoped I answered your question. If you want to yack, pm me your number.
(And no, you will never see any benefit forward of a beam reach...(Wind coming in 90º to the centerline of the boat)